Ch 2 · 2 Communication · AS & A Level Computer Science 9618
Topic 2 of 2 in Information Representation and Communication — AS & A Level Computer Science 9618 — 119 lessons.
Introduction
Almost every computer is part of a network. This lesson covers how networks are organised, the hardware and media that carry data, how Ethernet shares a cable, how streaming works, and how IP addresses and DNS find resources on the internet. (Syllabus topic 2: section 2.1.)
Key ideas — 2.1 (a) Why network?
Purpose and benefits: sharing resources (printers, internet connection, storage); sharing files and data; communication (email, messaging); central management (security, backups, software updates); users can log on from any computer.
🧑🏫 Teacher's explanation
Imagine a busy office building. If every employee had their own personal printer, scanner, and filing cabinet, the office would be cluttered, expensive, and impossible to manage. Instead, they use a shared printer in the hallway and a central filing system. This is the essence of a network: connecting independent computers so they can act as a single, efficient system.
📖 Definitions
🔍 Worked example — Easy
A small school wants to connect 30 computers in one computer lab to a single printer and a central file server. Identify the most appropriate network type (LAN or WAN) and state one benefit of this setup.
🔍 Worked example — Standard
Compare a LAN and a WAN in terms of ownership and media used.
🔍 Worked example — Exam-style
A multinational corporation has offices in London, New York, and Tokyo. They need to share a central database. Should they use a LAN or a WAN? Justify your answer.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(a) State one benefit of using a network instead of individual, standalone computers. [1] (b) The company decides to connect its branch office in another country to the headquarters. (i) Identify the type of network required for this connection. [1] (ii) Explain one difference between the media used in a LAN and the media used in this new network. [2] [4 marks]
Key ideas — 2.1 (b) Network models
Client-server: one or more servers provide services (files, email, printing, web pages, security, backup) to clients, which request them.
🧑🏫 Teacher's explanation
Imagine you are at a restaurant. In a client-server model, you are the customer (the client) and there is a waiter (the server). You cannot cook your own food; you must ask the waiter to bring it to you. This is great because the kitchen is organized and the chef controls the quality, but if the waiter disappears, no one gets food. In a peer-to-peer model, imagine a potluck dinner where everyone brings a dish and shares it directly with whoever is sitting nearby. There is no waiter, so if one person leaves, the party continues, but it is much harder to ensure everyone has a balanced meal or that the food is safe.
📖 Definitions
🔍 Worked example — Easy
A small home office has 3 computers used for basic word processing and printing. Which network model is most appropriate: client-server or peer-to-peer? Justify your answer.
🔍 Worked example — Standard
Compare the 'Single Point of Failure' risk between a client-server model and a peer-to-peer model.
🔍 Worked example — Exam-style
A company uses 'Thin Clients' for its customer service desk. What is the primary implication for the network connection?
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(a) Identify the network model being used. [1] (b) Explain one advantage and one disadvantage of this model for the university. [2] [3 marks]
Key ideas — 2.1 (c) Topologies
Packet transmission: in a bus, a packet is sent along the backbone and every device sees it, but only the one with the matching address accepts it. In a star, the switch reads the destination address and forwards the packet only to that device. In a mesh, the packet can travel by any available route.
🧑🏫 Teacher's explanation
Imagine you are in a classroom trying to pass a secret note. A bus topology is like everyone sitting in a single long row, and you pass the note from person to person along the line; if someone in the middle stops talking, the whole chain breaks. A star topology is like everyone having a direct private phone line to the teacher; if your phone breaks, everyone else can still talk to the teacher, but if the teacher's switchboard breaks, no one can communicate.
📖 Definitions
🔍 Worked example — Easy
A small home network consists of 4 computers connected to a single central switch. Identify the topology and state one advantage of this setup.
🔍 Worked example — Standard
A company requires a network that is highly fault-tolerant, meaning if one connection fails, data must be able to find an alternative route to its destination. Which topology should be used?
🔍 Worked example — Exam-style
Compare the packet transmission method in a Bus topology versus a Star topology.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(a) Identify the most suitable network topology for this requirement. [1] (b) Explain one disadvantage of the topology identified in part (a). [2] [3 marks]
Key ideas — 2.1 (d) Cloud computing
Cloud computing provides computing services (storage, processing power, applications) over the internet from remote servers.
🧑🏫 Teacher's explanation
Imagine you want to bake a cake. You could buy a massive, expensive oven, a huge mixer, and a pantry full of ingredients that you have to clean and maintain yourself (this is like owning your own physical servers). Or, you could use a professional community kitchen. You just walk in, use their high-end ovens and mixers, and pay only for the hours you actually use. If you suddenly need to bake 100 cakes, they have extra ovens ready for you. If you only bake one, you don't pay for the empty space.
📖 Definitions
🔍 Worked example — Easy
A small startup company needs to host a website. They have a very limited budget for hardware and do not have an IT department to maintain servers. Identify whether a public or private cloud is more suitable and state one benefit.
🔍 Worked example — Standard
A government agency handles highly sensitive citizen data. They require absolute control over where the data is stored and who can access the physical hardware. Should they use a public or private cloud? Justify your answer.
🔍 Worked example — Exam-style
A global streaming service experiences a massive spike in users during a major sporting event. Explain how cloud computing helps them manage this sudden increase in demand.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(a) State one benefit of using a public cloud for the customer-facing applications. [1] (b) Explain why the corporation might choose a private cloud for its internal databases instead of a public cloud. [2] [3 marks]
Key ideas — 2.1 (e) Wired and wireless media
Wired vs wireless: wired is faster, more reliable and more secure; wireless gives mobility, needs no cabling and is easier to expand, but has lower speed and range limits and needs encryption.
🧑🏫 Teacher's explanation
Think of data transmission like sending a message to a friend. Wired media is like sending a physical letter through a dedicated tube system; it is extremely fast, reliable, and private, but you have to build the tubes everywhere. Wireless media is like shouting through a megaphone; it is much easier to use and you can move around while doing it, but the message might be heard by others (security risk) or blocked by a thick wall (interference).
📖 Definitions
🔍 Worked example — Easy
A technician is choosing between copper and fibre-optic for a 500m connection. Compare the two media based on interference and speed.
🔍 Worked example — Standard
Calculate the bit rate if 125 Megabytes (MB) are transmitted in 10 seconds.
🔍 Worked example — Exam-style
A satellite link has a propagation delay of 250 ms. A microwave link has a delay of 5 ms. Explain why the satellite link has higher latency.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(a) State one advantage and one disadvantage of using a wireless network instead of a wired network. [2] (b) Explain why fibre-optic cable is preferred over copper cable for connecting two different buildings located 2km apart. [2] [4 marks]
Key ideas — 2.1 (f) LAN hardware
Router: connects different networks (e.g. a LAN to the internet); reads the destination IP address of each packet and forwards it towards its destination along the best route; can assign IP addresses (DHCP) and act as a gateway.
🧑🏫 Teacher's explanation
Imagine a large office building. If you want to send a memo to a specific person in a specific room, you don't just shout it in the hallway for everyone to hear. Instead, you use a specialized mailroom system.
📖 Definitions
🔍 Worked example — Easy
A technician needs to connect two separate LAN segments that both use the Ethernet protocol to create one single larger network. Which hardware device is most appropriate for this task?
🔍 Worked example — Standard
A network administrator is setting up a star topology. They need a device that can receive a data packet and ensure it is sent ONLY to the specific computer intended, rather than broadcasting it to every device on the network. Identify the device.
🔍 Worked example — Exam-style
A signal traveling through a long copper cable begins to degrade due to attenuation. Which hardware component must be placed at the midpoint to ensure the signal can reach the end of the cable?
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(a) Identify the hardware required to connect the two different networks. [1] (b) Identify the hardware required to allow wireless devices to connect to the wired network. [1] (c) Explain the role of a switch in a star topology LAN. [2] [4 marks]
Key ideas — 2.1 (g) Ethernet and CSMA/CD
Ethernet is the most common protocol for wired LANs. Data is sent in frames containing the source and destination MAC addresses.
🧑🏫 Teacher's explanation
Imagine a group of students in a quiet library trying to have a conversation. If everyone speaks at once, the noise becomes a jumble that no one can understand—this is a 'collision'.
📖 Definitions
🔍 Worked example — Easy
A device is about to transmit a frame on an Ethernet network. Describe the first step of the CSMA/CD process.
🔍 Worked example — Standard
Two devices on a shared Ethernet medium transmit data at the exact same time. Explain the sequence of events that follows the collision.
🔍 Worked example — Exam-style
A network uses CSMA/CD. Device A and Device B both detect a collision. They both wait for a random time t. If t_A = 5ms and t_B = 5ms, what is the most likely outcome when they attempt to re-transmit?
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(a) Identify the three main stages of the CSMA/CD process used when a collision occurs. [3] (b) Explain why it is necessary for devices to wait for a 'random' amount of time after a collision before attempting to re-transmit. [2] [5 marks]
Key ideas — 2.1 (h) Bit streaming
Bit streaming sends data (e.g. video or audio) as a continuous stream of bits that can be played while it is being received. A buffer stores some data ahead of playback, so short drops in speed do not interrupt it.
🧑🏫 Teacher's explanation
Imagine you are drinking water through a straw. If you drink at a constant, steady pace, the water flows smoothly into your mouth. This is like bit streaming: data flows in a continuous stream rather than arriving in one giant, heavy block.
📖 Definitions
🔍 Worked example — Easy
A user is watching an on-demand video with a bit rate of 2 Mbps. If the user's broadband speed is 5 Mbps, calculate the difference between the broadband speed and the bit rate in kbps.
🔍 Worked example — Standard
A live sports broadcast has a bit rate of 5 Mbps. A user has a connection speed of 4.5 Mbps. Explain what will happen to the playback.
🔍 Worked example — Exam-style
An on-demand movie file is 2 GB in size. It is being streamed at a constant bit rate of 4 Mbps. Calculate the approximate time in seconds required to stream the entire file.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
Key ideas — 2.1 (i) The internet and the WWW
Internet hardware: modems convert digital data to analogue signals for telephone lines and back; the PSTN (public switched telephone network) carries data over the traditional telephone system; dedicated (leased) lines give a permanent, private, high-speed connection; the cell phone network connects mobile devices through base stations (4G/5G).
🧑🏫 Teacher's explanation
Imagine the Internet is the global system of roads, highways, and tunnels that allow vehicles to travel anywhere in the world. It is the physical and logical infrastructure (the cables, routers, and satellites) that makes movement possible. The World Wide Web (WWW), however, is like the fleet of delivery trucks and the cargo they carry. The trucks use the roads to move specific goods (web pages, images, videos) to specific addresses. You cannot have the delivery service without the roads, but the roads themselves are not the delivery service.
📖 Definitions
🔍 Worked example — Easy
A user is browsing a website. Identify whether the following components belong to the 'Internet' or the 'WWW':
🔍 Worked example — Standard
A small business requires a connection that is never interrupted and provides consistent high speed, unlike a standard consumer line. Which type of internet connection should they use: PSTN or a Dedicated Line?
🔍 Worked example — Exam-style
A technician is setting up a remote site. They must convert digital signals from a router into analogue signals to transmit them over a standard telephone line. Name the hardware required and explain its function.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(a) Identify the type of internet connection the company should request. [1] (b) Explain the difference between the Internet and the World Wide Web. [2] [3 marks]
Key ideas — 2.1 (j) IP addressing
🧑🏫 Teacher's explanation
Imagine you are sending a letter to a student in a massive university campus. If you only wrote 'Room 4, Library', the postal service wouldn't know which university in the world you meant. You need a hierarchical address: Country arrow City arrow University arrow Building arrow Room. IP addressing works exactly like this. It provides a unique identifier for every device on a network so that data packets know exactly where to go.
📖 Definitions
🔍 Worked example — Easy
Identify the components of the IPv4 address 192.168.10.5 given a subnet mask of 255.255.255.0. State the Network ID and the Host ID.
🔍 Worked example — Standard
Convert the IPv6 segment 2001:0db8:0000:0000:0000:ff00:0042:8329 into its compressed form.
🔍 Worked example — Exam-style
A company uses a private IP range 10.0.0.x for its internal LAN. A device is assigned the IP 10.0.0.55. If the device needs to access a web server on the public internet, explain the role of the router.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(a) Explain one advantage of using private IP addresses instead of public IP addresses for internal devices. [2] (b) Describe the process that allows a device with a private IP address to communicate with a server on the public internet. [2] [4 marks]
Key ideas — 2.1 (k) URLs and DNS
A URL locates a resource on the WWW: protocol://domain name/path/file, e.g. https://www.codearc.net/learn/index.html.
🧑🏫 Teacher's explanation
Imagine you want to visit a friend named 'Alice'. You know her name, but to actually get to her house, you need her physical street address, like '123 Maple Avenue'. In the digital world, the URL is like the name 'Alice', and the IP address is the '123 Maple Avenue'. Computers are great at numbers but terrible at names; they need that numeric address to route data packets correctly.
📖 Definitions
🔍 Worked example — Easy
Identify the components of the following URL: https://www.example.com/images/logo.png
🔍 Worked example — Standard
A user enters http://study.cam.ac.uk/notes.pdf into a browser. Describe the role of the DNS in this process.
🔍 Worked example — Exam-style
Trace the sequence of events when a user requests https://www.google.com/search.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
Worked example
A company with 200 staff in one building wants a network. Recommend a model and topology, with reasons.
🔍 Worked example — Easy
A small home office has 3 computers. They need to share files occasionally and do not require centralized security or backups. Which model is most appropriate?
🔍 Worked example — Standard
A school computer lab has 30 workstations. All workstations are connected to a central switch. If one cable to a workstation breaks, the other 29 workstations continue to function. Identify the topology and state one benefit of this setup.
🔍 Worked example — Exam-style
A multinational corporation requires a network for 5,000 employees across different departments. They need strict security, centralized backups, and controlled software updates. Recommend a network model and a topology, justifying your choices.
⚠️ Common errors
Common mistakes
Practice
Answers: (1) Benefits: central security and backups; easy to manage many users. Drawbacks: servers are expensive and need specialist staff; server failure stops services. (2) A thin client depends on the server for processing and storage and needs a connection; a thick client processes and stores data itself and can work offline. (3) A device listens before sending; if a collision is detected while sending, it stops, sends a jam signal, and waits a random time before retrying. (4) Real-time streams a live event as it happens; on-demand streams stored files when requested and can be paused or skipped. (5) Private addresses are only used inside the LAN (and conserve public addresses); the router translates them to its public IP address (NAT), which also hides the device from the internet. (6) It translates the domain name into the IP address of the web server, so the browser can send its request to the right server.